Catalysis of Organic..

406“Green” Catalysts for BiodieselCO 2 + lightNo global warmingBiomassCO 2 releaseto atmosphereRefining / SynthesisUse in cars and trucksExploration1.55 kg fossil CO 2per 1 liter diesel burnedFigure 1. The life cycle of biodiesel compared to that of petroleum diesel.Another method to produce fatty esters is the by batch esterification of fattyacids, catalysed by H 2 SO 4 . The problem is that the batch operation mode againinvolves costly neutralization and separation of the homogeneous catalyst (5).Hence, the current biodiesel manufacturing is an energy intensive process thatconsumes large amounts of energy, primarily from fossil sources. Production costsand pollution produced by the present process may outweigh advantages of usingbiodiesel. Thus, biodiesel remains an attractive but still costly alternative fuel.During the last decade many industrial processes shifted towards using solidacid catalysts (6). In contrast to liquid acids that possess well defined acid properties,solid acids contain a variety of acid sites (7). Solid acids are easily separated fromthe biodiesel product; they need less equipment maintenance and form no pollutingby-products. Therefore, to solve the problems associated with liquid catalysts, wepropose their replacement with solid acids and develop a sustainable esterificationprocess based on catalytic reactive distillation (8). The alternative of using solid acidcatalysts in a reactive distillation process reduces the energy consumption andmanufacturing pollution (i.e. less separation steps, no waste/salt streams).Here we present the catalyst screening, highlight the pros and cons of severalsolid catalyst types, and discuss the possible applications of this novel process.Results and DiscussionIn large scale processes, a good esterification catalyst must fulfil several conditionsthat may not seem so important in the laboratory. The catalyst must be very activeand selective (by-products formed in secondary reactions are likely to render theprocess uneconomical), water-tolerant and stable at relatively high temperatures. Inaddition, it should be an inexpensive material that is readily available on anindustrial scale. Considering these conditions, we searched for a strong Brønsted acidwith increased hydrophobicity, and high thermal stability (up to 200–250 °C).Hydrophobic surfaces are preferable to avoid the covering with water of the solidacid surface and prevent the adsorption of organic materials.